1. Field of the Invention
The present invention relates to a diffuser panel and an optical apparatus which uses this diffuser panel.
2. Description of the Prior Art
A diffuser panel is an optical element for allowing rays incident in a certain direction to emerge in various directions. FIG. 26 shows a condition where incident rays are diffused by a transmission type diffuser panel 40. In this example, diffused rays are spread in a conical form. In this case, rays which have transmitted through the diffuser panel reach within a circular area D on a screen 41. A characteristic of a diffuser panel (hereinafter referred to as a diffusion characteristic) can be expressed by a diffusion angle representing a spread of the diffused rays and a brightness distribution of the rays within the circular area D. In addition, an area reached by diffused rays (the circular area D in the case of FIG. 26) is referred to as a diffusion area.
Known as a conventional examples of such a diffuser panel are diffuser panels which are disclosed by Japanese Patents Kokai Publication No. Sho 58-60642, No. Sho 57-148728, No. Sho 59-189330 and No. 2000-321626.
Diffusion characteristics of such diffuser panels are determined in most cases by surface configurations of the diffuser panels. Ground glass, for example, is conventionally known as a conventional diffuser panel which has fine convexities and concavities formed at random on a surface of a substrate. Furthermore, there is also known a diffuser panel which has micro lenses formed on a surface of s substrate.
These diffuser angels are used also in optical apparatuses. An example is a focusing panel which is used in an a single lens reflex camera. This focusing panel is disposed at a location which is optically equivalent to an image pickup surface in the camera.
In this single-lens reflex camera, rays which have passed through a photographic lens are incident on the focusing panel (diffuser panel). The rays which are incident on the focusing panel form an image of an object on this focusing panel. At this time, the rays which form the image of the object (incident rays) are diffused by the focusing panel and allowed to emerge as diffused rays. A diffusing surface of the focusing panel is configured so as to limit an angle within which the rays are diffused. Accordingly, the rays which form the image of the object are not diffused completely by the focusing panel. A photographer can therefore observe through a view finder the image of the object projected onto the focusing panel.
When the object is in focus (in a focused condition), a clear image is formed on the focusing panel. When the object is out of focus (defocused condition), in contrast, the image is blurred on the focusing panel. At this time, a blurred condition of the image is further emphasized by a diffusing function of the focusing panel. As a result, the photographer can confirm a composition and focused/defocused condition of the object.
From a viewpoint of the fact described above, it is required for a focusing panel to have characteristics to provide natural blurred impression, brightness, facilitation of focusing and so on. The blur, brightness and facilitation of focusing and so on are associated with a diffusion characteristic of the focusing panel (diffuser panel). A diffuser panel which has a larger diffusion angle, for example, more remarkably blurs an image as observed through a view finder. Accordingly, the differ panel which has the larger diffusion angle allows an image to be brought into focus easier but darkens an observed image.
The above expression “natural blur” means a condition where spot images are spread uniformly when an object is out of focus.
It is generally said that the diffuser panel disclosed by Japanese Patent Kokai Publication No. Sho 57-148728 provides natural blur. This diffuser panel has a surface in a sand-blasted condition. This surface in the sand-blasted condition is formed, for example, by dropping sand having a certain definite grain size onto a surface of a substrate in a definite amount per second over a sufficiently wide area.
Such a diffuser panel has a large diffusion angle. Furthermore, a brightness distribution within a diffusion area changes gently from a center toward a margin as shown in FIG. 27. Therefore, assume that a parallel light bundle is allowed to be incident on an entire surface of a diffuser panel 40 as shown in FIG. 28. In this case, rays which are allowed to emerge from the diffuser panel 40 are a total sum of diffused rays which have a diffusion characteristic shown in FIG. 27. Brightness is therefore uniform on a screen 41.
A diffuser panel which has such a diffusion characteristic provide favorable blur when the diffuser panel is used as a focusing panel. However, this diffuser panel has too large a diffusion angle, thereby darkening an image as observed through a view finder.
Known as conventional examples which have corrected a defect of darkening observed images are the diffuser panel (focusing panel) disclosed by the above-mentioned Japanese Patent Kokai Publication No. Sho 57-148728 and the diffuser panel (focusing panel) disclosed by the above-mentioned Japanese Patent Kokai Publication No. Sho 59-189330.
Out of these diffuser panels (focusing panels), the diffuser panel disclosed by Japanese Patent Kokai Publication No. Sho 57-148728 has a configuration shown in FIG. 30. Speaking concretely, this diffuser panel has a configuration in which micro lenses having a circular outer circumferential shape are arranged periodically on a surface of a substrate.
Furthermore, another conventional diffuser panel (focusing panel) uses micro lenses which have a regularly hexagonal outer circumferential shape and are periodically arranged with no gaps on a surface of a substrate as shown in FIG. 32.
These conventional diffuser panels (focusing plates) are capable of limiting spreads of diffusion angles (sizes of diffusion area). These diffuser panels are therefore capable of brightening images more easily than ground glass used as a diffuser panel.
An image of an object is formed on a focusing panel of a single-lens reflex camera as described above. However, objects to be photographed are not only those which have uniform brightness but also others which have various bright-dark patterns. Accordingly, rays at uniform brightness are rarely incident over an entire surface of focusing panel but bright and dark rays are incident mixedly on focusing panels.
FIG. 29 shows a condition where rays having bright and dark patterns are incident on a focusing panel 40. In this drawing, the incident rays are traced as a bright-dark lattice for simplicity. In this case, the rays are incident on a partial are of the focusing panel.
Let us assume, for example, that this partial area corresponds to eight micro lenses shown in FIG. 30. In this case, a brightness distributed is produced on a screen as shown in FIG. 31. In FIG. 28, bright portions are emphasized. A brightness distribution is therefore shown discretely as bright spots. However, brightness at a certain degree is present even among the bright spots. A brightness distribution formed by the bright spots is referred to as a diffusion pattern.
In a diffusion pattern shown in FIG. 31, bright areas (four bright spots located on right and left sides of three bright spots) are present around central portions (three bright spots in a vertical direction). Dark areas are present around these bright spots and bright spots are further present around the dark area. In other words, the diffusion patterns shown in FIG. 31 have a ring-like configuration. Accordingly, the focusing panel shown in FIG. 30 has is tends to blur an image of an object likes a dualized image.
Let us further assume that the partial area on which rays are incident corresponds to eight hexagonal micro lenses shown in FIG. 32. In this case, a brightness distribution is produced on the screen as shown in FIG. 33. Bright portions are emphasized also in FIG. 33. The brightness distribution is therefore shown discretely as bright spots. However, brightness at a certain degree is present even among the bright spots.
A diffusion pattern shown in FIG. 33 has bright ring shaped portions at locations a little apart from central portions. A focusing panel shown in FIG. 32 therefore provides unnatural blur like the focusing panel shown in FIG. 30.
An outer circumferential shape of a diffusion area which is rotationally asymmetrical with regard to a center is traced by connecting bright spots positioned at outermost locations. Triangular protruding areas are produced in an oblique direction in particular. Accordingly, blurred conditions of an image of an object may be different between a right-left direction and the oblique direction on a screen when the object is out of focus.
Furthermore, there is a conventional example which uses a irregular combination of lattice pitches like the focusing panel disclosed by Japanese Patent Kokai Publication No. 2000 321626. This conventional example is a focusing panel (diffuser panel) which has a random pattern. In this example, diffused rays are brightened by using three types of elements which have different heights.
However, it is not easy to manufacture this focusing panel (diffuser panel) since the panel requires forming random patterns on an entire surface. Furthermore, a diffraction grating has a rectangular shape, thereby allowing diffracted rays on high orders to produce flare on an entire screen.
The conventional diffuser panels which use the micro lenses are improved in brightness as compared with the diffuser panels composed of the sand-blasted surfaces as described above. However, the diffuser panel using the micro lenses provides diffused rays having a ring-like pattern or a diffusion area which is rotationally asymmetrical. Accordingly, this diffuser panel provides an unnaturally blurred image of an object which is out of focus when the diffuser panel is used as a focusing panel of an optical apparatus.